This invention relates to a method for manufacturing flat glass wherein the glass is formed into a flat sheet while supported on a surface of a pool of molten metal, commonly referred to as the float process. More particularly, this invention relates to a process for attenuating the glass while supported on the molten metal to a thickness below the equilibrium thickness of the glass in such a manner so as to minimize distortion in the product glass.
In a float forming process, molten glass is delivered onto a pool of molten metal and thereafter formed into a continuous ribbon or sheet of glass as disclosed, for example, in U.S. Pat. No. 710,357 of Heal; U.S. Pat. No. 789,911 of Hitchcock; U.S. Pat. Nos. 3,083,551 and 3,220,816 of Pilkington; and U.S. Pat. No. 3,843,346 of Edge et al. Under the competing forces of gravity and surface tension, the molten glass on the molten metal spreads outwardly to an equilibrium thickness of about 0.27 inches. In order to produce glass of thicknesses less than the equilibrium thickness, the prior art has resorted to various arrangements for stretching the glass while still in a viscous state on the pool of molten metal. The simplest stretching technique is that shown in U.S. Pat. No. 3,215,516 of Pilkington wherein stretching is done in the longitudinal direction (the direction of glass travel) only, wherein the stretching force is provided by the tractive means withdrawing the glass from the float chamber. In such an arrangement, the ribbon loses width as it becomes thinner. A common refinement of this arrangement is to employ lateral stretching means in order to reduce the loss of ribbon width as it is being stretched longitudinally. Typical of this latter approach is the process shown in U.S. Pat. No. 3,695,859 to Dickinson et al. Another approach is to maintain the ribbon of glass at essentially constant width by applying lateral tractive forces to edge portions of the ribbon as the ribbon is being attenuated in the longitudinal direction as exemplified in U.S. Pat. No. 3,843,346 of Edge et al.
Process perturbations originating with the attenuating process affect the topography of the glass ribbon in ways that degrade the optical quality of the product glass. The topography of float glass is characterized by two types of elongated features, thickness variations and corrugations, which extend generally parallel to the direction of glass travel, i.e., the longitudinal direction. These deviations from perfect flatness are, in effect, cylindrical lenses which distort light reflected from and/or transmitted through the product glass sheet. Analysis of the distortion patterns using optical scanners in a direction transverse to the direction that the glass traveled in the forming process reveals that the distortion patterns can be considered as consisting of randomly superimposed sinusoidal waves whose wavelengths vary over a wide range. It has also been found that the dominant component of the instrumentally measured signal corresponding to transmitted light occurs at rather well defined wavelengths that may range from about 1.2 to 1.4 inches (3.0 to 3.6 centimeters) for a "constant width" float forming process as in the Edge et al. patent cited above, to about 0.25 to 1.0 inches (0.6 to 2.5 centimeters) in the freefall type of float forming as in the Pilkington patents cited above. Furthermore, these dominant wavelengths have been found to lie within a range to which the human eye is particularly sensitive for most applications.
Surface distortion in float glass is believed to arise from several categories of perturbations. First, inhomogeneities in the glass composition ("ream") not only cause nonuniformity of the refractive index of the glass but also can contribute to surface distortion. Second, thermal nonuniformity either in the molten glass entering the float forming chamber or within the chamber itself can contribute to surface distortion. Third, variations in the flow of molten glass from the melter to the forming chamber, either volume flow rate fluctuations or inequalities in the thickness of entering molten glass across the width of the ribbon of glass. Fourth, mechanical perturbations from contact of various members of the forming apparatus with the deformable glass ribbon. These include, for example, the stretching machines and side barriers as well as fluctuations in the speed with which the dimensionally stable ribbon is withdrawn from the forming chamber. These perturbations in the glass/tin system generate thickness variations or corrugations in the glass through a variety of mechanisms such as differential stretching, viscous folding, wrinkling, embossing, and membrane stress. While minimizing the causes of these perturbations is desirable, such an approach is limited because the perturbations cannot be completely eliminated, particularly in the case where less than equilibrium thickness glass is being produced. Therefore, this invention relates to diminishing the effects on distortion of these perturbations rather than eliminating the perturbations themselves.
As a newly formed ribbon of glass still in a softened condition progresses along the molten tin bath its topography is continually changing as perturbations introduce new defects into the ribbon and previously introduced defects are changed in shape. A defect may decrease in amplitude by means of viscous decay, or the wavelength of a distortion pattern may be altered by extensive or compressive stresses. It would be desirable if these distortion decaying mechanisms could be coordinated with the attenuating process so as to minimize the amount of distortion imparted to the glass when attenuated to below equilibrium thicknesses.
Some attempts have been made in the prior art to correlate the manner of attenuation to minimizing surface defects such as in U.S. Pat. Nos. 3,440,030 (Thompson et al.); 3,533,772 (Itakura et al.); and 3,520,672 (Greenler et al.), but it is now believed that none of these approaches fully meets the problem.